This RENEWAL APPLICATION requests support for a cell-biophysical investigation of the underlying mechanism of cell division in sea urchin eggs. Unique proterties of local species will be exploited in an integrated experimental approach into the unsolved problems of cell cleavage by combining biophysical methods and experimental interventions. The goal is to determine the behavior and properties of the egg cortex which ultimately allow it to form and maintain a contracting cleavage furrow. Methods to be employed include cell elastimetry, cell electrophoresis, light microscopic determination of egg surface movements, and electron microscope analysis of egg surface topography. These will be applied to eggs under a wide variety of natural or experimental conditions in order to clarify cause from consequence in the cell cortex. A mechanical and structural profile of the egg cortex and at different times and places will be derived for the purposes of testing a new theory of cell cleavage. That theory emphasizes cortical contractility which is independent of the mitotic apparatus and cortical relaxation which is stimulated by the asters of the mitotic apparatus.